Media type sensing method for an imaging apparatus

ABSTRACT

A media type sensing method for an imaging apparatus includes the steps of changing a light intensity of a light source by changing a drive signal, while monitoring for an output change of a comparator; determining a drive signal value of the drive signal at a point of detection of the output change; and correlating the drive signal value to a specific media type.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an imaging apparatus, and, moreparticularly, to a method of media type sensing for an imagingapparatus.

[0003] 2. Description of the Related Art

[0004] Media sensors are used to detect the presence or absence of printmedia, and in some cases, are also used to determine the print mediatype. One form of a media sensor includes a single light source, such asa light emitting diode (LED), and a light detector, such as aphototransistor. Typically, the light detector is located on the sameside of a print media as the light source. During operation, the LEDdirects light at a predefined angle onto a material surface of the printmedia, and the surface characteristics of the print media are examinedin terms of the amount of light reflected from the surface that isreceived by the light detector. For example, the presence of the printmedia is detected based upon a predetermined amount of light reflectedfrom the media to the light detector.

[0005] One known sensor is a photo sensor that responds to a firstreference spectral reflection from a reference surface at an angle ofreflection equal to the angle of incidence. When a sheet of media isregistered by the printer against the reference surface, the photosensor responds to a second spectral reflection from the sheet of media.The ratio of the second spectral reflection intensity to first spectralreflection intensity is compared to a selected threshold to identify themedia by gloss level.

[0006] It is further known to have a detection system in which externallight is detected during a non-emission state of a light emittingelement and is used to correct a reference value. The correctedreference value is then compared to a detected value of light emittedfrom the light emitting element and reflecting from an original to becopied, and based on the comparison a determination is made as to thepresence of an original.

[0007] Some media sensors include a pair of light detectors, one of thelight detectors being positioned to sense reflected diffuse light and asecond detector positioned to sense reflected specular light. Such asensor may be used, for example, to detect and distinguish betweenvarious media types by optically measuring the glossiness of the mediabased on each of reflected specular light and reflected diffuselyscattered light. To measure the glossiness, a collimated beam of lightis directed towards the media and a reflectance ratio (R) of thedetected reflected specular light intensity and the detected diffusivelyscattered light intensity is calculated. The media sensor is initiallycalibrated by measuring a reflectance ratio (R0) on a known gloss media.A normalized reflectance ratio (Rn) is calculated using the formula:Rn=(R/R0). Normalized reflectance ratio Rn then is used to identify themedia type of an unknown media by a comparison of the normalizedreflectance ratio Rn to a plurality of normalized reflectance ratio Rnranges, each range being associated with a particular type of media.

[0008] What is needed in the art is an improved media sensing system andmethod that can use a simple single detector sensor using low costdigital electronics, and which reliably distinguishes between variousmedia types.

SUMMARY OF THE INVENTION

[0009] The present invention relates to an improved media sensing systemand method that can use a simple single detector sensor using low costdigital electronics, and which reliably distinguishes between variousmedia types.

[0010] The present invention, in one form thereof, is directed to amedia type sensing method for an imaging apparatus. The method includesthe steps of providing a sensor including a light source and a lightdetector; driving the light source based on a drive signal, the lightsource generating a light beam that impinges a print media sheet therebygenerating reflected light, the reflected light having a light intensityrelated to the drive signal and related to a type of the print mediasheet; detecting the light intensity of the reflected light with thelight detector, the light detector generating a detection voltage basedon the light intensity; providing a comparator to compare a referencevoltage to the detection voltage, wherein the comparator has an outputchange from an initial output state to a media type detection state whenthe detection voltage transitions across the reference voltage; changingthe light intensity of the light source by changing the drive signal,while monitoring for the output change of the comparator; determining adrive signal value of the drive signal at a point of detection of theoutput change; and correlating the drive signal value to a specificmedia type.

[0011] In another form thereof, the present invention is directed to amethod of correcting for sensitivity variation of media sensors. Themethod includes the steps of determining a first signal levelcorresponding to a first calibration media having a first glossiness;determining a second signal level corresponding to a second calibrationmedia having a second glossiness, the second glossiness being greaterthan the first glossiness; and determining a corrected normalized signallevel ratio of an unknown media based on the first signal level of thefirst calibration media and the second signal level of the secondcalibration media.

[0012] In yet another form thereof, the present invention is directed toa method of correcting for sensitivity variation of media sensors,including the steps of determining a first duty cycle corresponding to afirst calibration media having a first glossiness; determining a secondduty cycle corresponding to a second calibration media having a secondglossiness, the second glossiness being greater than the firstglossiness; and determining a corrected normalized duty cycle ratio ofan unknown media based on the first duty cycle of the first calibrationmedia and the second duty cycle of the second calibration media.

[0013] In yet another form thereof, the present invention is directed toa method of media type detection, including the steps of providing afirst signal source generating a first pulse width modulated signalhaving a duty cycle DS; providing a second signal source generating asecond pulse width modulated signal having a duty cycle DR; convertingthe first pulse width modulated signal to an output signal having avoltage level related to the duty cycle DS; converting the second pulsewidth modulated signal to a reference voltage related to the duty cycleDR; providing a sensor including a light source and a light detector;driving the light source based on the output signal, the light sourcegenerating a light beam that impinges a print media sheet therebygenerating reflected light, the reflected light having a light intensityrelated to the voltage level and related to a type of the print mediasheet; detecting the light intensity of the reflected light with thelight detector, the light detector generating a detection voltage basedon the light intensity; providing a comparator to compare the referencevoltage to the detection voltage, wherein the comparator has an outputchange from an initial output state to a media type detection state whenthe detection voltage transitions across the reference voltage; changingthe light intensity of the light source by changing the duty cycle DS ofthe first pulse width modulated signal, while monitoring for the outputchange of the comparator, wherein if after completing the step ofchanging the duty cycle DS the comparator has not experienced the outputchange, then changing the reference voltage by changing the duty cycleDR of the second pulse width modulated signal, and then repeating thestep of changing the duty cycle DS; determining a duty cycle valuecorresponding to the first pulse width modulated signal at a point ofdetection of the output change; and correlating the duty cycle value toa specific media type.

[0014] In still another form thereof, the present invention is directedto an apparatus for media type detection. The apparatus includes asignal source that generates a first pulse width modulated signal havinga duty cycle. A first filter circuit is electrically coupled to thesignal source for converting the first pulse width modulated signal toan output signal having a voltage level related to the duty cycle. Asensor includes a light source and a light detector. The light source iscoupled to the first filter for receiving the output signal. The lightsource generates a light beam that impinges a print media sheet therebygenerating reflected light. The reflected light has a light intensityrelated to the voltage level and related to a type of the print mediasheet. The light detector detects the light intensity of the reflectedand generates a detection voltage based on the light intensity. Avoltage reference source supplies a reference voltage. A comparator hasa first input port coupled to the voltage reference source and has asecond input port coupled to the light detector to compare the referencevoltage to the detection voltage. The comparator has an output changefrom an initial output state to a media type detection state when thedetection voltage transitions across the reference voltage. A controllercontrols the signal source to change the light intensity of the lightsource by changing the duty cycle of the first pulse width modulatedsignal. The controller is coupled to the output of the comparator tomonitor for the output change of the comparator. The controllerdetermines a duty cycle value (D) at a point of detection of the outputchange and correlates the duty cycle value (D) to a specific media type.

[0015] In still another form thereof, the present invention is directedto an imaging apparatus configured for media sensing. The imagingapparatus includes a media sensor including a light source and a lightdetector. A first signal source supplies a first pulse width modulatedsignal. A first low pass filter circuit is coupled between the firstsignal source and the light source. A comparator has a signal inputport, a reference input port and an output. The light detector iscoupled to the signal input port of the comparator. A second signalsource supplies a second pulse width modulated signal. A second low passfilter circuit is coupled between the second signal source and thereference input port of the comparator. A controller is coupled to theoutput of the comparator.

[0016] An advantage of the present invention is that it can use a simplesingle detector sensor using low cost digital electronics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above-mentioned and other features and advantages of thisinvention, and the manner of attaining them, will become more apparentand the invention will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

[0018]FIG. 1 is a diagrammatic representation of an imaging systemincluding an imaging apparatus embodying the present invention.

[0019]FIG. 2 is a side diagrammatic representation of a portion of theimaging apparatus depicted in FIG. 1.

[0020]FIG. 3 is circuit diagram showing an electrical circuit includingcomponents configured for implementing the present invention.

[0021]FIG. 4 depicts a flowchart of one media sensing method used inimplementing the present invention.

[0022]FIGS. 5A, 5B, 5C and 5D depict a flowchart of another mediasensing method used in implementing the present invention.

[0023] Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate embodiments of the invention, and such exemplifications arenot to be construed as limiting the scope of the invention in anymanner.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Referring now to the drawings, and particularly to FIGS. 1 and 2,there is shown an imaging system 6 embodying the present invention.Imaging system 6 includes a computer 8 and an imaging apparatus in theform of an ink jet printer 10. Computer 8 is communicatively coupled toink jet printer 10 via a communications link 11. Communications link 11may be, for example, a direct electrical or optical connection, or anetwork connection.

[0025] Computer 8 is typical of that known in the art, and includes adisplay, an input device, e.g., a keyboard, a processor, and associatedmemory. Resident in the memory of computer 8 is printer driver software.The printer driver software places print data and print commands in aformat that can be recognized by ink jet printer 10. The format can be,for example, a data packet including print data and printing commandsfor a given area, such as a print swath, and including a print headerthat identifies the swath data.

[0026] Ink jet printer 10 includes a printhead carrier system 12, a feedroller unit 14, a media sensor assembly 16, a controller 18, a mid-frame20 and a media source 22.

[0027] Media source 22, such as a paper tray, is configured and locatedto supply individual print media sheets 23 to feed roller unit 14, whichin turn further transports the print media sheets 23 during a printingoperation.

[0028] Printhead carrier system 12 includes a printhead carrier 24 forcarrying a color printhead 26 and a black printhead 28. A color inkreservoir 30 is provided in fluid communication with color printhead 26,and a black ink reservoir 32 is provided in fluid communication withblack printhead 28. Printhead carrier system 12 and printheads 26, 28may be configured for unidirectional printing or bi-directionalprinting.

[0029] Printhead carrier 24 is guided by a pair of guide members 34.Each of guide members 34 may be, for example, a guide rod or a guiderail. The axes 36 of guide members 34 define a bi-directional scanningpath 36 for printhead carrier 24. Printhead carrier 24 is connected to acarrier transport belt 38 that is driven by a carrier motor 40 via acarrier pulley 42. Carrier motor 40 has a rotating carrier motor shaft44 that is attached to carrier pulley 42. At the directive of controller18, printhead carrier 24 is transported in a reciprocating manner alongguide members 34. Carrier motor 40 can be, for example, a direct current(DC) motor or a stepper motor.

[0030] The reciprocation of printhead carrier 24 transports ink jetprintheads 26, 28 across the print media sheet 23, such as paper, alongbi-directional scanning path 36 to define a two-dimensional, e.g.,rectangular, print zone 50 of printer 10. This reciprocation occurs in amain scan direction 52. The print media sheet 23 is transported in asheet feed direction 54. In the orientation of FIG. 1, the sheet feeddirection 54 is shown as flowing down media source 22, and toward thereader (represented by an X) along mid-frame 20. Main scan direction 52,which is commonly referred to as the horizontal direction, is parallelwith bi-directional scanning path 36 and is substantially perpendicularto sheet feed direction 54, which is commonly referred to as thevertical direction. During each scan of printhead carrier 24, the printmedia sheet 23 is held stationary by feed roller unit 14.

[0031] Referring also to FIG. 2, feed roller unit 14 includes a feedroller 56 and corresponding pinch rollers 58. Feed roller 56 is drivenby a drive unit 60 (FIG. 1). Feed pinch rollers 58 apply a biasing forceto hold the print media sheet 23 in contact with respective driven feedroller 56. Drive unit 60 includes a drive source, such as a steppermotor, and an associated drive mechanism, such as a gear train orbelt/pulley arrangement. Feed roller unit 14 feeds the print media sheet23 along a print media path 55 in a sheet feed direction 54 (see FIGS. 1and 2).

[0032] Controller 18 is electrically connected to printheads 26 and 28via a printhead interface cable 62. Controller 18 is electricallyconnected to carrier motor 40 via an interface cable 64. Controller 18is electrically connected to drive unit 60 via an interface cable 66.Controller 18 is electrically connected to media sensor assembly 16 viaan interface cable 68.

[0033] Controller 18 includes a microprocessor having an associatedrandom access memory (RAM) and read only memory (ROM). Controller 18executes program instructions to effect the printing of an image on theprint media sheet 23, which can be one or more media types, such ascoated paper, plain paper, photo paper and transparency. In addition,controller 18 executes instructions to conduct media sensing, such asdetecting the presence or absence of the print media sheet 23, or thedetermination of media type, based on information received from mediasensor assembly 16.

[0034] It is contemplated that media sensor assembly 16 may bepositioned at any position along print media path 55. For example, mediasensor assembly 16 may be connected to printhead carrier 24 forreciprocation across the print media sheet 23 along bi-directionalscanning path 36 in print zone 50. As another example, print mediasensor assembly 16 may be located along print media path 55 near mediasource 22, as in the arrangement of FIG. 2.

[0035]FIG. 2 includes a broken out section that is enlarged in relationto the other components of FIG. 2 to more clearly show the components ofmedia sensor assembly 16. Media sensor assembly 16 is rotatably coupledto a frame 70 of ink jet printer 10. Also, media source 22 is attached,at least in part, to frame 70. Media source 22 includes a media support72 including a media support surface 74. In the embodiment shown in FIG.2, media sensor assembly 16 is located upstream of print zone 50, andmore particularly, adjacent to media source 22.

[0036] Media sensor assembly 16 includes a mounting device 78 and amedia sensor 80. Media sensor assembly 16 is coupled to frame 70 viamounting device 78. Mounting device 78 includes a pivot arm 82 that ispivotably attached to frame 70 via a pivot rod 84, and is pivotablyattached to media sensor 80 via pivot pins 86. A spring 90 provides abiasing force to pivot media sensor assembly 16 about an axis 92 in thedirection indicated by arrow 94. In an alternative arrangement, sensorassembly 16 may be biased simply by the forces of gravity. Thus,mounting device 78 is configured to facilitate movement of media sensor80 in a direction 88 toward print media path 55, and more particularly,toward media support 72, and to restrain movement of media sensor 80 insheet feed direction 54.

[0037] Media sensor assembly 16 includes a body 100 for supporting mediasensor 80, and may include at least one rotating member 102, such as forexample, one or more wheels. Media sensor 80 is positioned by mountingdevice 78 such that each rotating member 102 rotates due to contact witha surface 104 of print media sheet 23 as print media sheet 23 movesrelative to media sensor 80 in sheet feed direction 54 along print mediapath 55.

[0038] Contained within media sensor 80 of media sensor assembly 16 arethe electrical sensory components, such as for example, a light source,a specular detector and/or a diffuse detector, the configuration andoperation of which is known in the art. In its simplest form, the lightsource may include, for example, a light emitting diode (LED). In a morecomplex form, the light source may further include additional opticalcomponents for generating a collimated light beam. Each of the speculardetector and/or the diffuse detector can be, for example, aphototransistor. Advantageously, the present invention can use either asimple single detector sensor having only a specular detector, or amultiple detector sensor having both a specular detector and a diffusedetector.

[0039]FIG. 3 is circuit diagram showing an electrical circuit 110electrically coupled to controller 18 via interface cable 68. Electricalcircuit 110 includes media sensor 80, a comparator circuit 114, a mediasensor drive circuit 116, a first low pass filter circuit 118 and asecond low pass filter circuit 120.

[0040] Media sensor 80 includes a light source 122 and a specular(light) detector 124. In this embodiment, light source 122 is in theform of a light emitting diode (LED) and specular (light) detector 124is in the form of a phototransistor. As is known in the art.

[0041] Comparator circuit 114 includes a comparator U2A and its loadingresistors R3, R4, R8; positive feedback resistor R6; and pull-upresistor R5.

[0042] Media sensor drive circuit 116 may include a transistor and aload resistor.

[0043] First low pass filter circuit 118 includes resistor R2 andcapacitor C2.

[0044] Second low pass filter circuit 120 includes resistor R1 andcapacitor C1.

[0045] In the arrangement of electrical circuit 110, first low passfilter circuit 118 is coupled between the LEDPWM output port OUT1 ofcontroller 18 and media sensor drive circuit 116. Media sensor drivecircuit 116 is series coupled to light source 122 of media sensor 80.Light detector 124 of media sensor 80 is coupled to the non-invertinginput port (+) of comparator U2A. Second low pass filter circuit 120 iscoupled between the REFPWM output port OUT2 of controller 18 and theinverting input port (−) of comparator U2A. The output of comparator U2Ais coupled to an input port IN of controller 18. While for convenienceeach of the signal sources supplying pulse width modulated signalsLEDPWM and REFPWM are shown as being part of controller 18, it is to berecognized that one or both of the signal sources supplying pulse widthmodulated signals LEDPWM and REFPWM may be formed external to controller18.

[0046]FIG. 4 depicts a flowchart of one media sensing method used inimplementing the present invention, which will be described withreference to the circuit diagram of FIG. 3.

[0047] During operation of electrical circuit 110, at step S200,controller 18 generates a first pulse width modulated signal LEDPWM anda second pulse width modulated signal REFPWM. Each of the first pulsewidth modulated signal LEDPWM and the second first pulse width modulatedsignal REFPWM have a respective adjustable duty cycle.

[0048] At step S202, first low pass filter circuit 118 converts thefirst pulse width modulated signal LEDPWM to an output signal Vouthaving a voltage level related to the duty cycle of the first pulsewidth modulated signal LEDPWM. Likewise, second low pass filter circuit120 converts the second pulse width modulated signal REFPWM to areference voltage Vref having a voltage level related to the duty cycleof the second pulse width modulated signal REFPWM.

[0049] At step S204, output signal Vout is supplied to drive mediasensor drive circuit 116. Media sensor drive circuit 116 is configuredsuch that the voltage level of output signal Vout controls the currentpassing through light source 122, thereby in essence driving lightsource 122 based on output signal Vout. In turn, light source 122generates a light beam that impinges print media sheet 23 therebygenerating reflected light. The reflected light has a light intensitythat is related to the voltage level of output signal Vout and isrelated to a type of print media sheet 23, e.g., plain paper, photopaper, coated paper, transparency, etc.

[0050] At step S206, light detector 124 detects the light intensity ofthe reflected light, and generates a detection voltage Vdet based on thelight intensity.

[0051] At step S208, comparator U2A receives at its inverting input port(−) the reference voltage Vref, and receives at its non-inverting inputport (+) detection voltage Vdet. Since comparator U2A is configured as acomparator circuit 114, comparator U2A compares the reference voltageVref to the detection voltage Vdet. Comparator U2A has an output changefrom an initial output state, e.g., a digital low state (0) to a mediadetection state, e.g., a digital high state (1) when the detectionvoltage Vdet transitions across the reference voltage Vref. In circuit110 as configured in FIG. 3, the transition across the reference voltageVref is when the detection voltage Vdet exceeds reference voltage Vref,wherein for example the positive feedback arrangement of comparatorcircuit 114 amplifies even a slight voltage difference between detectionvoltage Vdet and reference voltage Vref to cause the output ofcomparator circuit 114 to switch from a digital low state (0) to adigital high state (1). It is to be understood, however, that ifalternatively comparator U2A received at its inverting input port (−)the detection voltage Vdet and received at its non-inverting input port(+) the reference voltage Vref, then the transition across the referencevoltage Vref is when the detection voltage Vdet exceeds referencevoltage Vref, wherein for example, the positive feedback arrangement ofcomparator circuit 114 amplifies even a slight voltage differencebetween detection voltage Vdet and reference voltage Vref to cause theoutput of comparator circuit 114 to switch from a digital high state (1)to a digital low state (0).

[0052] During media sensing, for example, controller 18 provides thefirst pulse width modulated signal LEDPWM with a low duty cycle, e.g.,0.5 percent, resulting in a detection voltage Vdet that has not yettransitioned across the reference voltage Vref. At step S210, controller18 then changes, e.g., increases, the light intensity of light source122 by changing the duty cycle of the first pulse width modulated signalLEDPWM that effectively drives light source 122, while monitoring forsaid output change of comparator U2A.

[0053] At step S212, controller 18 then determines the duty cycle value(D) for the first pulse width modulated signal LEDPWM at a point ofdetection of said output change, e.g., a change from a digital low state(0) to a digital high state (1) at comparator U2A.

[0054] At step S214, controller 18 then correlates the duty cycle value(D), either directly or indirectly, to a specific media type.

[0055] For example, duty cycle value (D) may be indirectly correlated toa particular media type by first forming a ratio of duty cycle value (D)with the duty cycle (D0) of pulse width modulated signal LEDPWMassociated with a known media, e.g., a calibration media. The use of acoated media as the calibration media results in a calibration to a lowglossy media. Thus, controller 18 can calculate a ratio D0/D, and thencorrelate the ratio D0/D to a specific media type. Alternatively, theratio could be D/D0.

[0056] To enhance the operation of the media sensing method of theinvention, correction can be made to compensate for such variables asthe differences between media sensor operation characteristics within areceived lot of media sensors, including for example, Media Sensor 1 andMedia Sensor 2, each of which could be used as media sensor 80. Table 1,below, is a comparison of exemplary data values associated with each ofMedia Sensor 1 and Media Sensor 2, wherein Media Sensor 1 is considereda “good” sensor and Media Sensor 2 is a “weak” sensor. Through thecompensation method, described below, Media Sensor 2 will be made to beeffectively a “good” sensor. TABLE 1 Exemplary values associated witheach of Media Sensor 1 and Media Sensor 2 for various media types.Sensor Sensor 1 Sensor 2 Sensor 2 Print Media Type 1 D D0/D D D0/DCalibration Media (coated) 45.50 1.00 72.00 1.00 Coated Paper I 36.751.24 60.18 1.20 Plain Paper I 29.25 1.56 48.30 1.49 Plain Paper II 25.671.77 42.44 1.70 Photo Paper I 10.25 4.44 18.54 3.88 Photo Paper II 6.586.91 12.16 5.92 Photo Paper III 6.50 7.00 12.09 5.96 Photo Paper IV 6.926.58 12.96 5.56 Transparency I 4.42 10.29 7.81 9.22 Transparency II 3.9211.61 7.59 9.49

[0057] Using the data from Table 1, media type determination criteriafor media type determination can be established, for example, as followsin Table 2. TABLE 2 Media Type Determination Media Type DeterminationCriteria Print Media Type D0/D < 1.4 Coated Paper 1.4 ≦ D0/D < 2.5 PlainPaper 2.5 ≦ D0/D < 9.0 Photo Paper 9.0 ≦ D0/D Transparency

[0058] Referring again to Table 1, as between Media Sensor 1 and MediaSensor 2, there can be significant variations in the normalized ratiosD0/D among various media sensors for certain media due to variations inoptical components and arrangements. For example, the print media PhotoPaper III can have normalized ratios from 6.0 to 7.0, depending on thecharacteristics of the particular media sensor used. This variation canbe removed by using a correction factor CF, as in the equation thatfollows.

[0059] The correction factor CF is determined by the equation:

CF=K/[(D0/Dh)−1]

[0060] wherein:

[0061] D0 is the duty cycle of the pulse width modulated signal LEDPWMassociated with media sensor 80 for low glossy calibration media, e.g.,coated media;

[0062] Dh is the duty cycle of the pulse width modulated signal LEDPWMassociated with media sensor 80 for high glossy calibration media, e.g.,transparency; and

[0063] K is the average of [(D0/Dh)−1] measured for multiple mediasensors, e.g., ten media sensors.

[0064] A high glossy calibration media may be, for example, atransparency.

[0065] Any normalized ratio (D0/D) for media sensor 80 will then becorrected by the equation:

Corrected Normalized Ratio (CRn)=[(D0/D)−1]×CF+1

[0066] FIGS. 5A-5D depict a flowchart of another media sensing methodused in implementing the present invention. This method compensates forbackground light that can possibly leak into media sensor 80, andcompensates for the turn on voltage of light source 122. Forconvenience, abbreviations are used in the flowchart of FIGS. 5A-5D, asfollows:

[0067] DS=Duty Cycle of LEDPWM

[0068] DR=Duty Cycle of REFPWM

[0069] S1=LEDPWM Duty Cycle Step, e.g., 0.5%

[0070] S2=REFPWM Duty Cycle Step, e.g., 2%

[0071] F0=15%, REFPWM Duty Cycle for a One-half Volt Step at Vref

[0072] DR0=REFPWM Duty Cycle Equivalent to Background Light

[0073] DS0=LEDPWM Duty Cycle at which LED Turns ON

[0074] At step S300, controller 18 switches light source 122, such as anLED, to an OFF state. In other words, the duty cycle DS of pulse widthmodulated signal LEDPWM is set to zero percent.

[0075] At step S302, controller 18 sets the duty cycle of pulse widthmodulated signal REFPWM to 60 percent, which corresponds to a referencevoltage Vref of 2.0 volts.

[0076] At step S304, controller 18 determines whether the output ofcomparator U2A is at a digital high state (1).

[0077] If the result at step S304 is YES, then at step S306 an errorindication that the background light level is too high is provided atone of printer 10 and computer 8.

[0078] If the result at step S304 is NO, then the process proceeds tostep S308.

[0079] At steps S308 and S310, controller 18 reduces the duty cycle DRof pulse width modulated signal REFPWM by an amount S2, which isequivalent to a REFPWM duty cycle step, e.g., two percent, and monitorsthe output of comparator U2A for an output change from a digital lowstate (0) to the digital high state (1) to determine the level ofbackground light that is sensed by media sensor 80.

[0080] At step S312, controller 18 then saves the current duty cycle DRof pulse width modulated signal REFPWM as DR0, which represents thepulse width modulated duty cycle equivalent to the background light.

[0081] At step S314, controller 18 sets the duty cycle DR of pulse widthmodulated signal REFPWM slightly above that of DR0, such as by DR0+S2,which in turn is filtered and supplied as reference voltage Vref to thenon-inverting port of comparator U2A, and the duty cycle DS of pulsewidth modulated signal LEDPWM is set to zero.

[0082] At steps S316 and S318, controller 18 increases the duty cycle DSof pulse width modulated signal LEDPWM, which results in a detectionvoltage Vdet signal being supplied to the non-inverting port ofcomparator U2A, and monitors the output of comparator U2A for an outputchange from a digital low state (0) to the digital high state (1), whenthe detection voltage Vdet signal is greater than the reference voltageVref, so as to determine the duty cycle DS0 at which light source 122turns ON.

[0083] When the result of step S318 is YES, then at step S320,controller 18 stores DS0.

[0084] At step S322, controller 18 sets the duty cycle DR of pulse widthmodulated signal REFPWM above that of DR0 in steps of F0 at the highestpossible level such that DR=(DR0+n ×F0)<100, wherein F0 is a 15 percentREFPWM duty cycle corresponding to a one-half volt step of referencevoltage Vref, and n is the highest possible step number, e.g., n=6.

[0085] At step S324, the duty cycle DS of pulse width modulated signalLEDPWM is set at the turn on level of light source 122, i.e., DS=DS0.

[0086] At step S326, the duty cycle DS of pulse width modulated signalLEDPWM is increased by a duty cycle step, S1, and is stored as the newvalue DS, such that DS=DS+S1. S1 may be, for example, a one-half percentduty cycle increase.

[0087] At step S328, it is determined whether the output of comparatorU2A has experienced a change in output state from low (0) to high (1);or, in other words, whether the detection voltage signal Vdet is greaterthan reference voltage Vref.

[0088] If at step S328 the determination is NO, the process proceeds tostep S330, where it is determined whether pulse width modulated signalLEDPWM has reached its maximum level.

[0089] If at step S330, the determination is NO, then the processreturns to step S326.

[0090] If at step S330 the determination is YES, the process proceeds tostep S332 wherein the duty cycle DR of pulse width modulated signalREFPWM is reduced by F0 by reducing the step number n by one count,i.e., n=n−1, and DR=DR−F0. The process then returns to step S324.

[0091] If, however, at step S328 the determination was YES, then theprocess proceeds to step S334.

[0092] At step S334, a duty cycle C for pulse width modulated signalLEDPWM that gives a signal equal to one unit reference voltage step ofreference voltage Vref is determined, wherein:

C=(DS−DS0)/n.

[0093] Thereafter, one of paths P336, P338 and P340, is performeddepending on whether it is desired to set a low normalization value, toset a high normalization value, or to perform media detection,respectively.

[0094] For low normalization (path P336), e.g., normalization based on alow glossy calibration media, such as coated paper, at step S336, C issaved as C0.

[0095] For high normalization (path P338), e.g., normalization based ona high glossy calibration media, such as a transparency, at step S338, acorrection factor CF is determined based on the equation:

CF=9/[(C0/C)−1]

[0096] wherein C is the duty cycle for pulse width modulated signalLEDPWM that gives a signal equal to one unit reference voltage step fora sensed sheet of calibration high glossy media.

[0097] For media detection (path P440), at step S340, a correctednormalized ratio CRn is determined by the equation:

CRn=[(C0/C)−1] ×CF+1,

[0098] wherein:

[0099] CRn is the corrected normalized duty cycle ratio of the unknownmedia;

[0100] C0 is a duty cycle of a first calibration media, e.g., a lowglossy media, for a unit reference voltage step corresponding to thechanging of the duty cycle DR;

[0101] CF is a correction factor based on a duty cycle of a secondcalibration media, e.g., a high glossy media, for the unit referencevoltage step corresponding to the changing of the duty cycle DR; and

[0102] C is a measured duty cycle of the unknown media for the unitreference voltage step corresponding to the changing of the duty cycleDR.

[0103] At step S342, corrected normalized ratio CRn is compared to themedia sensing ranges, such as those depicted in Table 3 below. TABLE 3Media Type Determination Media Type Determination Criteria Print MediaType CRn < 1.4 Coated Paper 1.4 ≦ CRn < 2.5 Plain Paper 2.5 ≦ CRn < 9.0Photo Paper 9.0 ≦ CRn Transparency

[0104] While this invention has been described with respect to variousembodiments, the present invention can be further modified within thespirit and scope of this disclosure. This application is thereforeintended to cover any variations, uses, or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of the appended claims.

What is claimed is:
 1. A media type sensing method for an imagingapparatus, comprising the steps of: providing a sensor including a lightsource and a light detector; driving said light source based on a drivesignal, said light source generating a light beam that impinges a printmedia sheet thereby generating reflected light, said reflected lighthaving a light intensity related to said drive signal and related to atype of said print media sheet; detecting said light intensity of saidreflected light with said light detector, said light detector generatinga detection voltage based on said light intensity; providing acomparator to compare a reference voltage to said detection voltage,wherein said comparator has an output change from an initial outputstate to a media type detection state when said detection voltagetransitions across said reference voltage; changing said light intensityof said light source by changing said drive signal, while monitoring forsaid output change of said comparator; determining a drive signal valueof said drive signal at a point of detection of said output change; andcorrelating said drive signal value to a specific media type.
 2. Themethod of claim 1, further comprising the steps of: providing a signalsource generating a first pulse width modulated signal having a dutycycle; converting said first pulse width modulated signal to an outputsignal having a voltage level related to said duty cycle; and using saidoutput signal as said drive signal.
 3. The method of claim 2, whereinsaid changing step changes said light intensity of said light source bychanging said duty cycle of said first pulse width modulated signal,while monitoring for said output change of said comparator.
 4. Themethod of claim 3, wherein said determining step determines a duty cyclevalue (D) of said first pulse width modulated signal at said point ofdetection of said output change.
 5. The method of claim 4, wherein saidcorrelating step correlates said duty cycle value (D) to said specificmedia type.
 6. The method of claim 5, further comprising the steps of:determining a duty cycle value (D0) of a calibration media; calculatinga normalized ratio of said duty cycle value (D) and said duty cyclevalue (D0); and correlating said normalized ratio to said specific mediatype.
 7. The method of claim 6, further comprising the step ofgenerating a correction factor-for said sensor using an equation:CF=K/[(D0/Dh)−1] wherein: D0 is the duty cycle associated with saidsensor for a low glossy calibration media; Dh is the duty cycleassociated with said sensor for a high glossy calibration media; and Kis the average of [(D0/Dh)−1] measured for multiple sensors.
 8. Themethod of claim 7, further comprising calculating a corrected normalizedratio (CRn) by an equation: (CRn)=[(D0/D)−1] ×CF+1
 9. The method ofclaim 1, wherein said sensor is located upstream of a print zone of saidimaging apparatus.
 10. The method of claim 1, wherein said sensor islocated in a print zone of said imaging apparatus.
 11. The method ofclaim 1, wherein said imaging apparatus is an ink jet printer.
 12. Themethod of claim 1, wherein said reference voltage is generated byproviding a second pulse width modulated signal, and converting saidsecond pulse width modulated signal to said reference voltage.
 13. Amethod of correcting for sensitivity variation of media sensors,comprising the steps of: determining a first signal level correspondingto a first calibration media having a first glossiness; determining asecond signal level corresponding to a second calibration media having asecond glossiness, said second glossiness being greater than said firstglossiness; and determining a corrected normalized signal level ratio ofan unknown media based on said first signal level of said firstcalibration media and said second signal level of said secondcalibration media.
 14. A method of correcting for sensitivity variationof media sensors, comprising the steps of: (a) determining a first dutycycle corresponding to a first calibration media having a firstglossiness; (b) determining a second duty cycle corresponding to asecond calibration media having a second glossiness, said secondglossiness being greater than said first glossiness; and (c) determininga corrected normalized duty cycle ratio of an unknown media based onsaid first duty cycle of said first calibration media and said secondduty cycle of said second calibration media.
 15. The method of claim 14,wherein said determining of said first duty cycle is performed by thefurther steps of: providing a signal source generating a first pulsewidth modulated signal having a variable duty cycle; converting saidfirst pulse width modulated signal to an output signal having a voltagelevel related to said variable duty cycle; providing a sensor includinga light source and a light detector; driving said light source based onsaid output signal, said light source generating a light beam thatimpinges said first calibration media thereby generating reflectedlight, said reflected light having a light intensity related to saidvoltage level and related to a type of said first calibration media;detecting said light intensity of said reflected light with said lightdetector, said light detector generating a detection voltage based onsaid light intensity; providing a comparator to compare a referencevoltage to said detection voltage, wherein said comparator has an outputchange from an initial output state to a media type detection state whensaid detection voltage transitions across said reference voltage;changing said light intensity of said light source by changing saidvariable duty cycle of said first pulse width modulated signal, whilemonitoring for said output change of said comparator; and determiningsaid first duty cycle value as a value of said variable duty cycle at apoint of detection of said output change.
 16. The method of claim 14,wherein said determining of said second duty cycle is performed by thefurther steps of: providing a signal source generating a first pulsewidth modulated signal having a variable duty cycle; converting saidfirst pulse width modulated signal to an output signal having a voltagelevel related to said variable duty cycle; providing a sensor includinga light source and a light detector; driving said light source based onsaid output signal, said light source generating a light beam thatimpinges said second calibration media thereby generating reflectedlight, said reflected light having a light intensity related to saidvoltage level and related to a type of said second calibration media;detecting said light intensity of said reflected light with said lightdetector, said light detector generating a detection voltage based onsaid light intensity; providing a comparator to compare a referencevoltage to said detection voltage, wherein said comparator has an outputchange from an initial output state to a media type detection state whensaid detection voltage transitions across said reference voltage;changing said light intensity of said light source by changing saidvariable duty cycle of said first pulse width modulated signal, whilemonitoring for said output change of said comparator; and determiningsaid second duty cycle value as a value of said variable duty cycle at apoint of detection of said output change.
 17. The method of claim 14,wherein step (c) is performed using the equation: CRn=[(D0/D)−1)×CF+1wherein: CRn is said corrected said normalized duty cycle ratio of saidunknown media; D0 is said first duty cycle of said first calibrationmedia; CF is a correction factor based on said second duty cycle of saidsecond calibration media; and D is a measured duty cycle of said unknownmedia.
 18. A method of media type detection, comprising the steps of:providing a first signal source generating a first pulse width modulatedsignal having a duty cycle DS; providing a second signal sourcegenerating a second pulse width modulated signal having a duty cycle DR;converting said first pulse width modulated signal to an output signalhaving a voltage level related to said duty cycle DS; converting saidsecond pulse width modulated signal to a reference voltage related tosaid duty cycle DR; providing a sensor including a light source and alight detector; driving said light source based on said output signal,said light source generating a light beam that impinges a print mediasheet thereby generating reflected light, said reflected light having alight intensity related to said voltage level and related to a type ofsaid print media sheet; detecting said light intensity of said reflectedlight with said light detector, said light detector generating adetection voltage based on said light intensity; providing a comparatorto compare said reference voltage to said detection voltage, whereinsaid comparator has an output change from an initial output state to amedia type detection state when said detection voltage transitionsacross said reference voltage; changing said light intensity of saidlight source by changing said duty cycle DS of said first pulse widthmodulated signal, while monitoring for said output change of saidcomparator, wherein if after completing said step of changing said dutycycle DS said comparator has not experienced said output change, thenchanging said reference voltage by changing said duty cycle DR of saidsecond pulse width modulated signal, and then repeating the step ofchanging said duty cycle DS; determining a duty cycle valuecorresponding to said first pulse width modulated signal at a point ofdetection of said output change; and correlating said duty cycle valueto a specific media type.
 19. The method of claim 18, further comprisingthe step of forming a corrected normalized duty cycle ratio of anunknown media based on said duty cycle value.
 20. The method of claim19, wherein said corrected normalized duty cycle ratio of said unknownmedia is determined using an equation: CRn=[(C0/C)−1)×CF+1 wherein: CRnis said corrected normalized duty cycle ratio of said unknown media; C0is a first duty cycle of a first calibration media for a unit referencevoltage step corresponding to said changing of said duty cycle DR; CF isa correction factor based on a second duty cycle of a second calibrationmedia for said unit reference voltage step corresponding to saidchanging of said duty cycle DR; and C is a measured duty cycle of saidunknown media for said unit reference voltage step corresponding to saidchanging of said duty cycle DR.
 21. An apparatus for media typedetection, comprising: a signal source that generates a first pulsewidth modulated signal having a duty cycle; a first filter circuitelectrically coupled to said signal source for converting said firstpulse width modulated signal to an output signal having a voltage levelrelated to said duty cycle; a sensor including a light source and alight detector, said light source being coupled to said first filter forreceiving said output signal, said light source generating a light beamthat impinges a print media sheet thereby generating reflected light,said reflected light having a light intensity related to said voltagelevel and related to a type of said print media sheet; said lightdetector detecting said light intensity of said reflected and generatinga detection voltage based on said light intensity; a voltage referencesource for supplying a reference voltage; a comparator having a firstinput port coupled to said voltage reference source and having a secondinput port coupled to said light detector to compare said referencevoltage to said detection voltage, wherein said comparator has an outputchange from an initial output state to a media type detection state whensaid detection voltage transitions across said reference voltage; and acontroller that controls said signal source to change said lightintensity of said light source by changing said duty cycle of said firstpulse width modulated signal, said controller being coupled to saidoutput of said comparator to monitor for said output change of saidcomparator, said controller determining a duty cycle value (D) at apoint of detection of said output change and correlating said duty cyclevalue (D) to a specific media type.
 22. The apparatus of claim 21,wherein said reference voltage is generated by providing a second pulsewidth modulated signal, and providing a second filter circuit forconverting said second pulse width modulated signal to said referencevoltage.
 23. The apparatus of claim 21, wherein said controllerdetermines a duty cycle value (D0) of a calibration media; calculates anormalized ratio of said duty cycle value D and said duty cycle valueDO; and correlates said normalized ratio to a specific media type. 24.The apparatus of claim 21, wherein said apparatus is an ink jet printer.25. An imaging apparatus configured for media sensing, comprising: amedia sensor including a light source and a light detector; a firstsignal source for supplying a first pulse width modulated signal; afirst low pass filter circuit coupled between said first signal sourceand said light source; a comparator having a signal input port, areference input port and an output, said light detector being coupled tosaid signal input port of said comparator; a second signal source forsupplying a second pulse width modulated signal; a second low passfilter circuit coupled between said second signal source and saidreference input port of said comparator; and a controller coupled tosaid output of said comparator.
 26. The imaging apparatus of claim 25,wherein said first signal source and said second signal source areincluded in said controller.